Optically connectable circuit board with optical component(s) mounted thereon

ABSTRACT

An optically connectable circuit board and optical components mounted thereon. At least one component includes optical transceivers and provides an optical connection to the board. Electronic components may be directly connected to the board electrically or optically. Also, some electronic components may be indirectly connected optically to the board through intermediate optical components.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is related to U.S. patent application Ser. No.10/305,516 entitled “HIGH SPEED DATA CHANNEL INCLUDING A CMOS VCSELDRIVER AND A HIGH PERFORMANCE PHOTODETECTOR AND CMOS PHOTORECEIVER” toBoszo et al., and U.S. patent application Ser. No. 10/31,585 entitled“BACKPLANE ASSEMBLY WITH BOARD TO BOARD OPTICAL INTERCONNECTIONS” toBoszo et al., both filed coincident herewith and both assigned to theassignee of the present invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to high-speed inter chip opticalconnections and more particularly to high speed optical inter boardconnections between logic and/or memory chips on different printedcircuits, e.g., connected to a backplane.

2. Description of the Related Art

FIG. 1 shows an example of a state of the art electro-optical assembly100 with a passive backplane 101 connecting two circuit boards 103 eachwith mounted electro-optical components 105. The boards 103 pass signalsto each other over the passive backplane 101 through connectors 107.Chips 108, 110, 112, 114 populate and are packaged in the opticalcomponent modules 105.

FIGS. 2A–B show an example of typical orthogonal cross sections of thegeneral board structure 200 of either/both of the backplane and circuitboards. This passive board structure 200 includes both electrical wiringchannels 203 and optical wiring channels 205. A dielectricbackplane/board material 201 provides a mechanical structure formaintaining and protecting the embedded copper wiring infrastructure andpower distribution on wiring channels 203. Wiring channels 203 provideelectronic signal media in the X and Y dimensions with interlayer orinterlevel vias (not shown) connecting electrical signals betweendifferent wiring layers.

On one surface of the backplane/board are optical wave guides 205, whichare shown here in a single layer. These optical wave guides 205 can be asuitable polymer or glass material deposited on the preexisting surfaceof the backplane/board material, or it can be an independentlymanufactured structure containing polymers or glass or optical fibers,that is laminated onto the board material. A fill material 207 separatesthe optical wave guides. The fill 207 provides isolation and planarity.

So, from FIG. 1 typical losses in a chip-to-chip (e.g., 108–112) opticalpath crossing the backplane 101 can be determined. In this example, theonboard path may be 50 centimeters for each board 103, with the boardsspaced apart on the backplane 101 by 1 meter. The optical material is apolymer, for example. A typical board polymer exhibits a 0.03 dB/cm lossand a typical backplane polymer exhibits a 0.05 dB/cm loss. A typicalchip to board coupling loss is 3 dB and a typical board to backplaneconnector loss is 2 dB. Thus, for this path, the signal loss is 18 dB.

This 18 dB loss is substantial and, remembering that each 3 dB dropcorresponds to a loss of halving the signal, corresponds to a sixty fourtime signal reduction, i.e., the receiver signal at chip 114 is 1/64 thestrength at chip 108. So, to compensate for an 18 dB loss thetransmitted signal at chip 108 must have 64× the signal required at thereceiver chip 114. This is an unacceptable power requirement,particularly when tens of signals are required for a typical data pathand well in excess of what is usually allowed for data communicationsoptical paths.

There are a number of known approaches to driving down these losses.Chip-to-board coupling losses can be reduced with better electro-opticalpackaging. Better materials can be used to reduce Channel losses, e.g.,laminating fibers into the board (instead of depositing a polymer) is acostly approach to making channel losses negligible. Finally, improved(and more expensive) connectors can reduce board-to-backplane couplingloss. Connector losses result primarily from mechanical mismatches andso, can be improved by reducing tolerances, e.g., with precisionmechanical machining. Unlike material changes (e.g., in the channels),precision mechanical machining requires new and better tools andprocessing, which is not an incremental cost increase. Each of thesethree state of the art approaches produce incremental improvements onlywith solving difficult engineering problems accompanied by sometimesdramatic cost increases. It may be possible using some combination ofthese approaches to reduce the loss of the above example from 18 dB toan acceptable level, e.g., 9 dB or an 8× reduction from the transmittedsignal to the receiver.

FIG. 3 shows an example of a multidrop backplane 300, e.g., in a largeswitch or a server backplane. There may be thousands of such signals ona typical such backplane 300. Such a multidrop backplane 300 isparticularly suited for servers to bus or distribute (multidrop) thesignals, i.e., to fan out each transmitted signal in parallel tonumerous (e.g., 8, 16, or even 32) boards 302 connected to the backplane300.

However, with the boards 302 connected to “tap points” along thebackplane optical channels, some signal is lost at each tap point. So,if each “tap point” causes a few dB signal drop from the originallytransmitted signal strength (a 3 dB drop per tap point is quiteoptimistic), adding 3 boards to the improved path increases the totalsignal loss back to 18 dB. Clearly, the added work and expense has notprovided for inclusion of more than a few more boards. For thousand ofsignals (instead of tens of signals), the total power required isprohibitive.

Furthermore, such a 4 to 5 board system would be inflexible, unscalablebeyond 5 boards. Likewise removing 1 or 2 boards for a midrange systemwould not scale particularly easily either. Signal integrity andradiation issues would arise in the infrastructure which is designed forthe 4–5 board system.

Thus, there is a need for an assembly including a backplane withmultiple boards optically connected together for use in a large switchor in a server. There is a further need for such an assembly that may beconstructed from a wide range of wave guide materials and in particular,those that are tolerant of channel loss. Further, there is a need forsuch an assembly that is tolerant of mechanical misalignment, therebyavoiding a requirement for precise mechanical alignment (i.e., that istolerant of large coupling loss in the board-to-backplane connectors).There is also a need for such an assembly that allows multidroppingsignals transmitted from one board, so that multiple boards can receivethe signal. Finally, there is a need for a scalable assembly that allowsfor a wide range of system scaling (i.e., a few boards to many boards)on a single physical infrastructure or backplane.

SUMMARY OF THE INVENTION

It is a purpose of the present invention to improve systemcommunications;

It is yet another purpose of the invention to improve onboardcommunications.

The present invention relates to an optically connectable circuit boardand optical components mounted thereon. At least one component includesoptical transceivers and provides an optical connection to the board.Electronic components may be directly connected to the boardelectrically or optically. Also, some electronic components may beindirectly connected optically to the board through intermediate opticalcomponents.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of illustrativeembodiments of the invention with reference to the drawings, in which:

FIG. 1 shows an example of a state of the art electro-optical assemblywith a passive backplane connecting two circuit boards each with mountedelectro-optical components;

FIGS. 2A–B show an example of typical orthogonal cross sections of thegeneral board structure 200 of either/both of the backplane and circuitboards;

FIG. 3 shows an example of a multidrop backplane, e.g., in a largeswitch or a server backplane;

FIG. 4 shows an example of a backplane assembly 400 according to aembodiment of the invention;

FIG. 5 illustrates fan-out on the self-contained backplane of FIG. 4;

FIG. 6 is a plot of achievable number of boards N vs. percent ofoutcoupled power per grating;

FIG. 7 shows an example of a cross section of a grating structure forcoupling transceiver optics in a chip to an optical channel on a boardor backplane according to a preferred embodiment of the presentinvention;

FIG. 8 shows an example of a preferred gating structure and chip mountedon a board structure, e.g., a backplane;

FIG. 9 shows an example of an alternative embodiment grating structure;

FIG. 10 shows an example of a structure 1000 for coupling an opticalsignals from external light source/sink to the on-backplanetransceivers;

FIG. 11 shows an example backplane attachment structure;

FIG. 12 shows a male optical plug 1201 inserted into the female flangedstructure;

FIG. 13 shows an example of a board-to-backplane connector assemblyaccording to a preferred embodiment of the present invention;

FIG. 14 shows an example of a preferred board attached to the backplane;

FIG. 15 shows an example of a multi-channel transceiver chip mounted ona preferred embodiment board;

FIG. 16 shows an example of a second multi-channel transceiver assemblymounted on a preferred embodiment board;

FIG. 17 shows an example of a preferred embodiment board or circuit cardwith representative simple optical wiring;

FIGS. 18A–B show a top view and a cross-sectional view of an example ofa mounted preferred embodiment onboard transceiver chip;

FIG. 19 shows a preferred embodiment wherein optical wave guides arealigned in different directions (e.g., perpendicular to each other)within the board;

FIG. 20 shows a variation on the embodiment of FIG. 19 for chips(processors, logic or memory) with integrated active optical elements;

FIG. 21 shows a schematic representative of a worst case system path.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Turning now to the drawings and, more particularly, FIG. 4 shows anexample of preferred embodiment boards 401 mounted on a backplaneassembly 400 such as is described in U.S. patent application Ser. No.10/317,585 entitled “BACKPLANE ASSEMBLY WITH BOARD TO BOARD OPTICALINTERCONNECTIONS” to Boszo et al. filed coincident herewith, assigned tothe assignee of the present invention and incorporated herein byreference. Each preferred embodiment board 401 is mounted on andoptically connected to the backplane 403 through an optical transceiver405. Also, as further described hereinbelow, chips may be mounted on apreferred embodiment substrate or interposer and are optically connectedto the preferred embodiment board 401. The optical transceiver 405receives inputs and repeats or relays the received input optical signalsusing its own active circuitry. The repeated signals are transmitted tothe transceiver outputs.

The optical transceivers 405 isolate all board losses from the backplanelosses, thereby making each of the board design specifics irrelevant toand independent of the backplane design and vice-versa. Thus, theonboard losses are self-contained within each board 401 and do not addto the backplane losses. Likewise, backplane losses are self-containedwithin the backplane 403 and do not affect board losses. Thus, fan-outon the backplane 403 is a self-contained and manageable design problem.Also, very lossy connectors can be used to connect the boards to thetransceivers 405, since the connector loss is isolated and so, notadditive to the backplane loss.

FIG. 5 illustrates fan-out on the self-contained backplane 403 of FIG.4. A laser source 501 is shown at one end 501 (i.e., on an unshown boardlocated there) of the backplane 403 for the worst-case optical path 505in this N-board system example. The worst case path 505 spans the entirebackplane 403, incurring the maximum possible channel loss, fanning outto all of the remaining N-1 transceivers 405 along the channel or path505.

So, for example, a photodetector in each transceiver may require a 20 μWoptical signal to sense the signal properly, e.g., at several GHz. Witha 3 dB backplane-to-photodetector loss in the grating coupling 503,optical power to the end or Nth transceiver must be at least 40 μW atthe far end 502 of the board 403. The optical gratings 503 are identicaland each outcouples something less than 100% of the power in thechannel, i.e., some portion (X %) is outcoupled. So, for an N boardsystem, the link budget must accommodate N-1 grating losses (i.e.,(N-1)*X %) plus the 3 dB channel loss. For a 10 mW laser 501 at 40%quantum yield and with a 3 dB coupling loss to the backplane channel505, delivers 2 mW to the channel 505 directly under the laser 501 atthe left end 509. Thus, Table 1 shows an example of a link budget forthis example for different values of X, in this example for X=2, 4, 6,8, and 10. Channel loss outcoupling amounts are compared for each valueagainst how much power is required to reach the far end and the totallink budget for 2 mW (2000 μW) at the source.

TABLE 1 Link Budget Calculation % Outcoupled Corresponding Power Powerper Loss per grating Required at the Link Budget with Grating (dB) FarEnd (μW) 2000 μW at Source 2 0.09 2,000 0 4 0.18 1,000 3 6 0.27 667 4.88 0.36 500 6 10 0.46 400 7

FIG. 6 is a plot of achievable system size (number of boards N) vs.percent of outcoupled power per grating (X % which is a designparameter) based on Table 1 for three examples. In the lowest curve 600,the channel is 1 meter with a 0.03 dB/cm channel loss and a maximumsystem size of 10 boards. This 10 board maximum is achieved with thegratings designed for 10–12% outcoupling. The middle curve 602 shows alossless channel material (e.g., fiber) has an 18 board maximum systemsize with gratings designed for 6% outcoupling. Alternately, this 18board maximum can be achieved with the middle curve 600 by doublingsource laser power, e.g., by using 2 lasers instead of 1. The highestcurve 604 shows a channel with both a lossless material and source laserpower doubled achieves a 35 board system maximum with gratings having2–4% outcoupling.

FIG. 7 shows an example of a cross section of a grating structure 700for coupling transceiver optics in a chip 701 to an optical channel on aboard or backplane according to a preferred embodiment of the presentinvention. The chip 701 contains a laser 703 that transmits light in onedirection, a laser 705 that transmits light in the opposite directionand a photodiode 707 receiving and detecting laser energy from otherboards. The two lasers 703, 705 are driven by the same chip signal (notshown). Two lasers 703, 705 accommodate internal backplane boardpositions, i.e., those that are not at either end of the backplane. Onelaser 703 transmits to boards to one side, e.g., its left, and the otherlaser 705 transmits boards on its other side, i.e., to its right. Thephotodiode 707 senses light traveling in the channel that is outcoupledfrom the backplane (not shown) by the grating structure 700 to the chip701.

In this example, a tapered matched-index layer 709 is insulated by alow-index material 711. Mirrors 713 direct incident laser light from thechip 701 to either side (e.g., leftwards or rightwards) into the channel(not shown). A grating 715 in the matched-index layer 709 is designed toprovide the desired amount of outcoupling as provided above in Table 1and FIG. 6. Power connections 717 connect power from the backplane orboard (not shown) to the optoelectronics circuits on the chip 701.

FIG. 8 shows an example of a preferred gating structure 700 and chip 701mounted on a board structure 800, e.g., a backplane. The gratingstructure 700 provides coupling between the chip 701 and the opticalchannel 802 in the board structure 800. It should be noted that thephotodiode 707 does not sense light transmitted from the same chip 701because, the mirrors 713 direct the light away from the grating 715(i.e., to the left and the right) and the photodiode 707 is in thecenter of the chip 701 above the grating 715.

FIG. 9 shows an example of an alternative embodiment grating structure900 with like elements labeled identically. In this embodiment 900, thelasers 703, 705 are in the center of the chip 901 and a pair ofidentical photodiodes 902, 904 and gratings 906, 908 are located oneither side of the lasers 703, 705. The two photodiodes 902, 904 arewired together (not shown) to act as a single photodiode. The advantageof this embodiment is that the photodiodes 902, 904 can sense light inthe channel that was transmitted by this same chip, which may be used intesting. When a board is inserted into the backplane, a continuity checkcan be done using this embodiment.

FIG. 10 shows an example of a structure 1000 for coupling an opticalsignals from external light source/sink to the on-backplane transceivers701. In this example, a second chip 1003 is flip mounted back to backwith on-backplane transceivers 701. The second chip 1003 also contains alaser 1013 and a photodiode 1009 and a solder interface 1005 connects itthrough vias 1007 to the first chip 701. Through vias 1007 providedpower to the top chip 1003 and pass selected electrical signals betweenthe two chips 701, 1003.

The photodetector 1009 on the top transceiver chip 1003 detects light1011 from an external source, e.g., from a connected board or chip. Thetop photodetector 1009 converts the external light into an electricalsignal and relays the electrical signal through the vias 1007 to drivers(not shown) for the lasers 703, 705 in the bottom chip 701. The lasers703, 705 in the bottom chip 701 converts the electrical signal to anoptical signal to recreate the optical signal, which is relayed to thebackplane channel (not shown in this example) as previously described.

Signals in the opposite direction originate when the photodetector 707on the bottom transceiver chip 701 detects/senses light in the backplanechannel (not shown). The photodetector 707 converts the detected lightinto an electrical signal. The electrical signal passes back overthrough vias 1007, to a driver (not shown) for the laser 1013 in the toptransceiver chip 1003. The laser 1013 in the top chip 1003 recreates theoptical signal, and relaying the optical signal 1015 to an externalsink, e.g., to a board.

FIG. 11 shows an example backplane attachment structure or backplaneoptical socket 1100. In this example, the dual-chip electro-opticaltransceiver 1000 is connected to a backplane 403 and “potted” into aflanged structure 1105 for easy attachment with an optical plug that istolerant of fairly crude alignment.

FIG. 12 shows a male optical plug 1201 inserted into the female flangedstructure of the backplane optical socket 1100. The plug 1201 containsfibers 1203, 1205 carrying optical inputs 1203 and optical outputs 1205.The plug 1201 mechanically butts against the top chip 1003 of thedual-chip transceiver structure 1000. Optionally, this butted connectionforms a raw optical interface 1209 that can be enhanced with opticalgels. Thus, provided that the fiber loss is negligible, only theinterface 1209 is lossy in the connection. It should be noted that theplug 1201 and cable 1203, 1205 can be plugged into a circuit board thatis plugged into this same backplane 403 as further described hereinbelowor, the cable 1203, 1205 can run to another backplane (not shown) toextends the present invention to multiple frames, if losses permit.

FIG. 13 shows an example of board-to-backplane connector assembly 1300mounted on a preferred embodiment 401 according to the presentinvention. A board-backplane optical jumper 1310 includes a pair ofplugs 1201 and 1302 attached to either end of optical cables 1203, 1205and connects the preferred circuit board 401 to a backplane (not shownin this example). Spring clamps on board optical sockets 1303 hold theboard plug 1302 in place to provide an optical connection to an onboardtransceiver structure 1304 as described hereinbelow and, substantiallysimilar to a transceiver, e.g., 1000 in FIG. 10. A spring 1309 attachesacross the optical jumper 1310 to provide tension for adequate opticalcoupling and to maintain plug 1201 inserted into a backplane opticalsocket.

FIG. 14 shows an example of a preferred board 401 attached to thebackplane 403. The board 401 is inserted on an edge into a typicalelectrical edge connector 1409 on the backplane 403. An optical jumper1310 in a connector/cable assembly 1300, optically connects the board401 to the backplane 403. The spring 1309 in the connector/cableassembly 1300 is mounted on the board 401 and forcibly holds thebackplane plug 1201 in backplane optical socket 1100. Preferably, whenthe board 401 is inserted in the electrical edge connector 1409, eachbackplane plug 1201 automatically mates with an optical connector 1100in the backplane optical socket 1100 making the optical connections. Theoptical signal repeats in both transceivers 1000, 1304 such that thetotal connection loss is due to the 2 raw interfaces at the plugs 1201,1302. Since the plug-to-plug link budget can be ample (3–6 dB or evenlarger, if needed), the mechanical tolerances can be loose, and the costof these plug and flange structures can be very low. It should be notedthat each connector/cable assembly 1300 can be used for a parallel businterconnection. For example, with a linear array of lasers on 125micron centers, a 1 inch wide plug having 2 rows of fibers could easilyaccommodate 80 signals in and 80 signals out. This can be used toimplement an 8-byte bus with parity and control signals as discussedhereinbelow. Such a plug would have a form-factor and tolerance similarto a phone jack.

FIG. 15 shows an example of a first preferred embodiment onboardmulti-channel transceiver chip 1500 mounted on a board 1501 providing anoptical interface to an optical connector at one end of an opticaljumper, e.g., 1302 in FIG. 13. Thus, this onboard multi-channeltransceiver chip 1500 provides an indirect optical connection to theboard 1501 for an optical jumper. Optical signals from an optical jumperat the top surface 1502 of the multi-channel transceiver chip 1500 areprovided to optical transceivers 1504 which convert the optical signalsto electrical signals. The electrical signals pass on vias 1506 throughthe multi-channel transceiver chip 1500 to a suitable I/O chip attach,e.g., solder balls 1508, shown in further detail in inset 1510. Thepreferred board 1501 includes wiring and power layers 1512, 1514oriented perpendicularly to each other, analogous to backplane wiringdescribed in FIGS. 2A–B. Metal pads 1516 in a chip attach location onthe board 401 are connected (not shown) to appropriate wiring and powerlayers 1512, 1514. The preferred board 401 is substantially similar inconstruction to the backplane.

It should be noted that the onboard multi-channel transceiver chip 1500of this example is shown mounted on a printed circuit board 1501 thatmay not have onboard optics and is used as an optical connection to theboard. In particular, the onboard multi-channel transceiver chip 1500can attach such a board to an optical backplane 400, especially where itis desirable to provide optical signals directly to the board instead ofthrough an edge connector, e.g., 1409 in FIG. 14. It should be notedthat if there is no onboard optical interconnection, the multi-channeltransceiver chip 1500 may be mounted on the backplane and, theelectrical signals provided through an edge connector to the board. Itshould also be noted that an optical jumper connection to an opticalbackplane is described for example only. Any suitable optical connectionmay be used to connect any suitable optical source/sink, e.g., toanother frame or backplane.

FIG. 16 shows an example of a second preferred embodiment onboardmulti-channel transceiver assembly 1600 mounted on a preferredembodiment board 401. In this embodiment, the multi-channel transceiverassembly 1600 is optically coupled to board optical channels 1602 in thesurface 1604 of the preferred embodiment board 401. Thus, this onboardmulti-channel transceiver chip 1600 has a direct optical connection tothe board 401. The multi-channel transceiver assembly 1600 issubstantially a parallel multi-signal version of the single back to backchip assembly of chips 701, 1003 of FIG. 10. Optical signals arereceived by the top transceiver chip 1606 and relayed as electricalsignals to the lower chip 1608 over through vias 1610. Opticaltransceivers 1612 on the lower chip 1608, shown in detail in inset 1614,convert the optical signals from through vias 1610 back to opticalsignals which are passed through on-chip optical coupling 1616 torespective optical channels 1602. Similarly, optical signals from theboard follow a reverse path and are optically transmitted out the toptransceiver chip 1606 into a cable (not shown). Power and groundcontacts 1620, 1622 may be provided outboard of lower chip 1608 to board401.

FIG. 17 shows an example of a preferred embodiment board 401 or circuitcard with representative simple optical wiring 1702. In this example,the board 401 includes a single layer of straight optical wires orchannels 1700. Optical board I/Os enter and leave the board 401 at anoff-board transceiver assembly 1600. In this embodiment, the transceiverassembly 1600 optically communicates with a number of onboardtransceivers 1704 over the optical wiring channels 1700. Further,off-board transceiver assembly 1600 is an optical to optical interface,while onboard transceivers 1704 are each optical to electricalinterfaces. So, in this example, the off-board transceiver assembly 1600only connects an external entity, e.g., the backplane, to the onboardtransceivers 1704 and is isolated from other chips 1706 on the board401. The onboard transceivers 1704 are optically connected to each otherand to the off-board transceiver and are electrically connected to otherchips 1706 on the board 401.

In this example, electrical wiring (not shown) for logic and memory inchips 1706 may be contained within localized board areas, with chips1706 within a particular area interconnected with short electrical wires(not shown). For longer paths between areas (e.g., paths that musttraverse a major portion of the board), the logic or memory chips 1706communicate through one of the onboard transceiver chips 1704. Thus,electrical paths between chips 1706 is contained to a localized area ora short electrical connection 1708 to an onboard transceiver chip 1704.So a signal from a chip on one end of the board 401 to the other, passeselectrically to an onboard transceiver 1704, optically between onboardtransceivers 1704 and, passes electrically from the onboard transceiver1704 to the receiving chip. Thus, the path includes only a shortelectrical connection between the onboard transceivers 1704 and theorigination/destination logic or memory chips 1706. Advantageously, thisembodiment provides a very simple optical infrastructure (e.g., a singlestraight layer of channels) and, the logic and memory chips 1706 may bestate of the art CMOS chips with only electrical I/Os. Only thetransceiver chips have optical components. Further, special chippackaging is not required for the preferred board of this example.

FIGS. 18A–B show a top view and a cross-sectional view through B—B of anexample of a preferred embodiment onboard transceiver chip 1704 mountedon a preferred embodiment board 401. The parallel optical channels 1702pass beneath the chip 1704 and electrical (e.g., solder) connections1800 are interdigitated with respect to the optical channels 1702.Optical signals between the transceiver chip 1704 and the opticalchannels 1702 pass through an optical interface 1802 located between theoptical channels 1702 and respective chip elements 1804 (i.e., lasersand photodiodes) The electrical connections 1800 may be wire bonds,soldered pads or solder balls or any other suitable board to chip directattach technology. It should be noted that soldering mechanically alignsthe chip 1704 with respect to the optical channels 1702.

FIG. 19 shows a preferred embodiment assembly for high performanceoptical interconnection of state of the art chips to the board 1900 andwherein optical wave guides 1902, 1904 are aligned in differentdirections (e.g., perpendicular to each other) within the board 1900. Inthis embodiment one or more chips 1906, such as a state of the art CMOSprocessor or CMOS memory is mounted on an active optical interposer1908. The chip(s) 1906 are attached to the optical interposer 1908 witha typical chip attach technology, e.g., controlled collapsible chipconnect (C4). A chip underfill 1910, e.g., epoxy, strengthens theattachment. Metal vias 1912 through the optical interposer 1908 passelectrical signals from the chip(s) to the board 1900 and to the activeinterposer elements (lasers 1914 and photodiodes 1916). Electricalsignals and power and ground are passed to the optical interposer 1908at mounting pads 1918. The optical interposer 1908 is attached at themounting pads 1918 using a suitable attach technology such a ball gridarray (BGA) attach. Light guide structures 1920 between the opticalinterposer 1908 and the board 1900 optically couple the activeinterposer elements to optical wave guides 1902, 1904. Since underfillis not required beneath the optical interposer 1908, the secondarysolder process attaching the optical interposer 1908 to the board 1900can be at relatively low temperature. Preferably, the light guidestructure is of a flexible optical transmissive material, e.g., atransparent gum, rubber, plastic or glass or, are a compliant (semifluid) forming beads as shown in FIG. 19. Optionally, however, lightguide structures 1920 may be prefabricated solid structures, e.g., glassbeads that are halved and glued to the board 1900. This embodiment hasparticular application to high performance systems, e.g., wherein stateof the art memory chips are closely coupled to microprocessors withmulti-GHz operating clocks to maximize system performance.

FIG. 20 shows a variation on the embodiment of FIG. 19 for chips(processors, logic or memory) with integrated active optical elements.An example of active optical elements integratable on state of the artCMOS is provided in U.S. patent application Ser. No. 10/305,516 entitled“HIGH SPEED DATA CHANNEL INCLUDING A CMOS VCSEL DRIVER AND A HIGHPERFORMANCE PHOTODETECTOR AND CMOS PHOTORECEIVER” to Boszo et al., filedcoincidentally herewith, assigned to the assignee of the presentinvention, and incorporated herein by reference. In this embodiment,optical vias 2000 are drilled (or etched) through the optical interposer2002 and filled with a matched-index material (e.g., glass). The opticalinterposer 2002 may be completely passive. Preferably, for betteroptical performance, the via ends are shaped into lenses 2004 using anyof several known methods. In this embodiment, the underfill epoxy 1910is optional and, if used, must also be a good optical match.Furthermore, chips with active optical may be mounted directly on theboard 1900 using well known direct chip attach technologies incombination with the above described optical transceiver mountingtechniques.

FIG. 21 shows a schematic representative of a worst case system path2100, e.g., for the backplane assembly 400 of FIG. 4 with boards 401such as the example in FIG. 17 mounted thereon. An electrical signaloriginating in inverter 2102 is converted to light in a first laser 2104in an off-board transceiver assembly on a first Board, e.g., Board 1 inFIG. 5. The light passes through an optical jumper 2106 to a backplanetransceiver 2108, e.g., chip 1003 in FIG. 10. A photodetector 2110converts the optical energy to electrical, which is amplified byamplifier 2112. The output of amplifier 2112 is converted back to lightin laser diode 2114. The laser diode 2114 drives an optical channel 2116in a backplane, e.g., 401 described hereinabove, which in this exampleis 1 meter long. A photodetector 2118 in another transceiver 2120 at theother end of the optical channel 2116 converts the optical energy fromthe backplane optical channel 2116 to electrical energy that isamplified by amplifier 2122. The output of amplifier 2122 is convertedback to light in laser diode 2124. The laser diode 2124 drives anotheroptical jumper 2126 connected to an off-board transceiver assembly on areceiving board (e.g., Board N in FIG. 5) at the other end of thebackplane optical channel 2116. A photodetector 2128 in the receivingboard off-board transceiver assembly converts received optical energy toelectrical, provided sufficient optical energy arrives. The electricalenergy from photodetector 2128 is amplified by amplifier 2130 anddistributed by the off-board transceiver assembly to onboardtransceivers.

Thus, in this example there are 6 signal conversions and 1 meter oftransit. Each of the conversions takes on the order of 10 picosecondsand the transit time is roughly 5 nanoseconds. Thus, the end-to-endlatency is dominated by transit time and roughly 5 nanoseconds. Channelfrequency is limited by the response of the slowest amplifier in thepath and/or, for a parallel bus, the skew between signals.

So for an 8-byte bus example provided hereinabove, the transceivers forall bits of the 8 byte quanta reside on the same chip minimizingresponse variation and skew. Further, for a parallel bus application,the signals should be sent source-synchronously, i.e., with anaccompanying clock signal as one of the spare bus signals. Furthermore,because the electro-optical devices and amplifiers response is in the 10s of picoseconds, this arrangement can readily accommodate signals ofseveral Ghz (perhaps 10 Ghz) without resorting to exotic signalingtechniques. Also, at these operating speeds, channel latency will beseveral cycles because latency is dominated by transit time, 5nanoseconds in this example.

Latency that is several cycles long poses a challenging arbitrationproblem for a shared bus implementation. Specifically, between twoboards on the backplane, the signal latency is primarily determined bythe physical distance on the backplane between the two boards. As can beseen from the above examples, this distance range from a inches foradjacent boards (hence a cycle or two) to as much as a meter (10 s ofcycles). Therefore, when the boards in the shared bus system all vie forthe bus, the requesting signals arrive at different times at the busarbitrator (the board selected for making all arbitration decisions),i.e., depending on where each of the requesting boards reside no thebackplane. Further, different boards may see the order of arrivalsdifferently. Since each of the boards most likely will not see therequests in the same consistent order, arbitration protocol is requiredto guarantee that the arbitration logic makes consistent bus grantdecisions.

For example, N backplane physical channels of the control channels areallocated for a “bus request” signal for each board. Each “bus request”signal is an assert only signal, i.e., it is asserted (e.g., carryingoptical energy) only when a board is requesting the bus. Further, itremains asserted until bus control is granted to the requesting board.Typically, the arbitrator or arbitration master board (e.g., thephysically center most board on the backplane) grants board requestsconsistent with the observed order of receiving requests. Each board(other than the arbitrator) is assigned an identification or bus grantID. The arbitrator grants bus control by selecting the bus grant ID forone of the boards, e.g., by providing the ID on a log₂(N)+1 bus grantchannel dedicated to bus grant signaling, e.g., by optically signalingthe grant ID in hexadecimal. Likewise, the arbitrator synchronizesarriving bus grant IDs on the 80-pin bus with a source-synchronous clockthat arrives at the boards with the bus grant IDs.

Advantageously, the present invention addresses all of the problemsfound in state of the art systems. In particular, the present inventionis directed toward a large switch or server environment in which thereare multiple boards connected to a backplane. The present inventionallows for a wide range of wave guide materials (i.e., is tolerant ofchannel loss) and does not require precise mechanical alignment (i.e.,is tolerant of large coupling losses in the board-to-backplaneconnectors). The present invention allows multidropping signalstransmitted from one board, so that multiple boards can receive thesignal and at a wide range of system scaling (i.e., a few boards to manyboards) based on a single physical infrastructure (backplane).

Having thus described preferred embodiments of the present invention,various modifications and changes will occur to a person skilled in theart without departing from the spirit and scope of the invention. It isintended that all such variations and fall within the scope of theappended claims. Examples and drawings are, accordingly, to be regardedas illustrative rather than restrictive.

1. An optically connectable circuit board comprising: a printed circuitcard with a plurality of chip attach locations; a plurality of opticalchannels in a surface layer of said printed circuit card; an electroniccomponent mounted in each of said chip attach locations and grouped intolocal area groups; a plurality of optical components including at leastone onboard transceiver, the at least one said onboard transceivercomprising a plurality of optical transceivers, each of said pluralityof optical transceivers optically connected to a corresponding opticalchannel, each of said local area groups being indirectly opticallyconnected to said printed circuit board through an electricallyconnected one of said plurality of onboard transceivers; and at leastone of said plurality of optical components includes an off boardtransceiver having a plurality of said optical transceivers, each havingan optical connection to one of said plurality of optical channels. 2.The optically connectable circuit board as in claim 1, wherein said offboard transceiver comprises: a first surface matable to a multi-signaloptical connector, said optical connection being through said firstsurface; said plurality of optical transceivers being at said firstsurface; and said at least one electronic component being attached to achip attach location at a second surface.
 3. The optically connectablecircuit board as in claim 1, wherein each of said optical transceiverscomprises: a photodiode detecting an incoming optical signal; and alaser diode sending an outgoing optical signal.
 4. The opticallyconnectable circuit board as in claim 1, wherein each of said local areagroups communicates off-board through said electrically connectedonboard transceiver and then, through said off board transceiver.
 5. Theoptically connectable circuit board as in claim 2, wherein said offboard transceiver provides an indirect optical connection, said secondsurface includes a plurality of chip pads, each of said plurality ofoptical transceivers being electrically connected to a corresponding oneof said chip pads.
 6. The optically connectable circuit board as inclaim 3, wherein said printed circuit card further comprises a pluralityof internal wiring layers.
 7. The optically connectable circuit board asin claim 3, wherein said off board transceiver further comprises: aplurality of second optical transceivers at said second chip surface,said plurality of second transceivers optically connected to acorresponding optical channel.
 8. The optically connectable circuitboard as in claim 3, wherein said off board transceiver is a pair oftransceiver chips mounted back to back, optical signals being relayedbetween corresponding optical transceivers on said first surface andsaid second surface, said one electronic component coupling saidplurality of optical channels off board for connection to a mated saidmulti-signal optical connector.
 9. The optically connectable circuitboard as in claim 5, wherein said off board transceiver furthercomprises a solder ball at each of said plurality of chip pads, saidsolder balls attaching said off board transceiver to said chip attachlocation.